1. Technical Field
The present invention relates to rotors for electric rotating machines that are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of Related Art
Interior Permanent Magnet (IPM) motors, such as the one disclosed in Japanese Patent Application Publication No. 2006-254599, generally have a plurality of permanent magnets embedded in a rotor core thereof along the circumferential direction of the rotor core.
FIG. 9 shows the overall configuration of a conventional IPM motor 10. As shown in the figure, the motor 10 includes a rotating shaft 11, a rotor 14 and a stator 18.
The rotor 14 includes a hollow cylindrical rotor core 12 and a plurality of permanent magnets 13. The rotor core 12 is formed by laminating a plurality of annular magnetic steel sheets in the axial direction and coaxially fixed on the rotating shaft 11. The permanent magnets 13 are embedded in the rotor core 12 so as to form a plurality of magnetic poles which are spaced in the circumferential direction of the rotor core 12 at predetermined intervals and the polarities of which alternate between north and south in the circumferential direction.
The stator 18 includes a hollow cylindrical stator core 17 and a stator coil 16. The stator core 17 has a plurality of slots (not shown) that are formed in the radially inner surface of the stator core 17 and spaced in the circumferential direction of the stator core 17 at predetermined intervals. The stator core 17 is coaxially disposed radially outside the rotor core 12 with a predetermined annular gap formed between the rotor core 12 and the stator core 17. The stator coil 16 is mounted on the stator core 17 so as to be partially received in the slots of the stator core 17.
Referring now to FIG. 10, the rotor core 12 has a plurality of pairs of through-holes 12a that are formed in the vicinity of the radially outer periphery of the rotor core 12. Each of the through-holes 12a extends in the axial direction of the rotor core 12 to penetrate the rotor core 12. The pairs of the through-holes 12a are spaced in the circumferential direction of the rotor core 12 at predetermined intervals. Moreover, each pair of the through-holes 12a is arranged so as to form a substantially truncated V-shape that opens toward the radially outer periphery of the rotor core 12. Each of the permanent magnets 13 is held in a corresponding one of the through-holes 12a of the rotor core 12 so as to extend in the axial direction of the rotor core 12. Moreover, for each pair of the through-holes 12a of the rotor core 12, the two permanent magnets 13 which are respectively held in the pair of the through-holes 12a together form one of the magnetic poles on the radially outer periphery of the rotor core 12. Further, when viewed along the axial direction of the rotor core 12, the two permanent magnets 13 are symmetrically arranged and extend obliquely with respect to a centerline C1 of the magnetic pole which bisects the magnetic pole in the circumferential direction of the rotor core 12. In addition, the rotor core 12 further has a plurality of pier portions 12b each of which is formed to extend radially inward from an annular portion of the rotor core 12, in which the permanent magnets 13 are embedded, along the centerline C1 of a corresponding one of the magnetic poles.
With the above configuration of the rotor 14, it is possible to utilize reluctance torque that is generated due to the anisotropy in magnetic reluctance of the rotor 14.
However, in the rotor 14, the magnetic flux generated by each pair of the permanent magnets 13 will leak radially inward (i.e., toward the rotating shaft 11) via the corresponding pier portion 12b of the rotor core 12, as indicated with arrowed dashed lines Y1 in FIG. 10. Consequently, the available reluctance torque will be reduced.
To solve the above problem, it is possible to configure a rotor 24 as shown in FIG. 11. Specifically, in the rotor 24, for each pair of the permanent magnets 23, there is formed a large opening 22c radially inside the central portion of the magnetic pole made up of the pair of the permanent magnets 23. The opening 22c extends in the axial direction of the rotor core 22 so as to penetrate the rotor core 22 and has substantially the same angular range as the pair of the permanent magnets 23. Consequently, with the opening 22c, the magnetic reluctance of the rotor 24 at the central portion of the magnetic pole is increased, thereby increasing the available reluctance torque.
However, with the above configuration, during rotation of the rotor 24, for each of the openings 22c, a radially-outer beam portion 22e of the rotor core 22 which is positioned radially outside the opening 22c will be moved radially outward by the centrifugal force. Consequently, it is impossible to keep the annular air gap formed between the rotor 24 and the stator (not shown) of the motor at the predetermined value.
To keep the annular gap at the predetermined value, referring further to FIG. 12, it is possible to provide, for each of the openings 22c, a pier portion 22f for reinforcing the rotor core 22. The pier portion 22f radially extends along a centerline of the opening 22c which bisects the opening 22c in the circumferential direction of the rotor core 22, thereby connecting a pair of radially-inner and radially-outer beam portions 22d and 22e of the rotor core 22 that are respectively positioned radially inside and outside the opening 22c. 
However, with the above pier portion 22f, there is the same problem as with the pier portion 12b of FIG. 10. That is, the magnetic flux generated by the corresponding pair of the permanent magnets 23 will leak radially inward via the pier portion 22f, thereby reducing the available reluctance torque.
In addition, to reduce the magnetic flux leakage via the pier portion 22f, one may consider reducing the circumferential thickness of the pier portion 22f. However, in this case, the strength of the rotor core 22 would be accordingly reduced, thereby making it difficult for the rotor core 22 to withstand the centrifugal force during rotation of the rotor 24.